The bovine herpes virus Type-1 (BHV-1), an alpha herpes virus, is an important etiological agent of respiratory (infectious bovine rhinotracheitis; IBR), and genital, (infectious pustular vulvovaginits; IPV), diseases in cattle (Gibbs and Rweyemamy, 1977; Schwyzer and Ackermann, 1996). BHV-1 infection costs $100 million to Canadian and up to $500 million to American cattle industry. BHV-1 is also associated with bovine respiratory disease complex (BRDC) resulting from subsequent secondary bacterial infections and costs the U.S. cattle industry up to 3 billion dollars annually (Jones and Chowdhury, 2007). This hampers dairy and beef trade with BHV-1 free countries for instance those within the European Union, where BHV-1 eradication efforts are being made.
The respiratory form of the disease spreads via aerosols and is characterized by rhinotracheitis, conjunctivitis, and development of bovine respiratory disease (BRD) complex complicated by secondary bacterial infections. The predisposition to bacterial complications by the virus is related directly to its cytolytic effect on the cells of nasal and tracheal mucosa apart from its immunosuppressive effects. The genital infection spreads via genital secretions, semen and foetal fluids and is manifested as IPV, balanoposthitis, endometritis and abortions (Yates, 1982; Tikoo et al., 1995; Thiry et al. 2006). During the abortion storm that commonly follows respiratory and conjunctival disease, up to 60% of herd may abort due to this virus. The BHV-1 may also be transmitted via contaminated cryo-preserved semen during artificial insemination (Jones, 1998, 2003). The virus replicates in local mucus membranes and neurons of trigeminal and sacral ganglia where it survives in the host as latent infection but gets activated during stressful conditions such as transportation and parturition.
Such disease outbreaks are due to lack of an effective vaccine result in viral latency and viral shedding. This necessitates the use of antibiotics to prevent secondary bacterial infections that lower the quality of milk and beef.
The currently used inactivated and modified live vaccines (MLVs); (Van Donkersgoed et al, 1991; van Drunen Littel-van den Hurk, 2006) do not confer adequate protection against BHV-1 infection. The MLVs not only result in viral latency but also cause abortions in pregnant animals (Van Donkersgoed and Klassen, 1995). Prior immunization of calves against BHV-1 does not reduce the risk of infection since viral outbreaks have been reported in feed lot calves in Canada (Van Donkersgoed and Klassen, 1995).
Since no effective vaccines are available to prevent latency and viral shedding in a herd, passive immunization with virus neutralizing antibodies could provide an effective adjunct approach for prevention and control of BHV-1 infection in addition to conventional immunization. The monoclonal IgG1 antibody that neutralizes BHV-1 virus has been developed which is capable of providing protective immunity (Levings and Stoll, 1991).
The hetero-tetrameric immunoglobulin molecule provides remarkable intra-molecular synergy in the context of antigen recognition by the antigen binding fragment (Fab) at the amino terminal end as well as biological effector functions via crystallizable fragment (Fc) at the carboxy terminal end. Antibodies provide the most successful class of targeted therapeutics in addition to their application in specific clinical or immunodiagnosis of various diseases. Antibody engineering has its origins in hybridoma technology which converts B lymphocytes from an immunized animal or subject into hybrid cell lines that have acquired the ability to produce monoclonal antibodies (Kohler and Milstein, 1975). Indeed, monoclonal antibodies have become indispensable in immunodiagnostics together with significant therapeutic potential via innovative recombinant DNA technologies such as chimerization and humanization of antibodies (Morrison et al., 1984; Boulianne et al, 1984). The isolation of antibodies from hybridomas, however, has its limitations with regard to stability and expression level. These limitations can be circumvented by developing combinatorial libraries of single chain variable fragment of an antibody (scFv) or as fragment antigen-binding (Fab) that could be successfully expressed in microrganisms (Winter et al., 1994; Harvey et al., 2004). Such a dissection of antibodies into minimal antigen binding fragments has certain advantages as these can be fused with a range of molecules including toxins for the treatment of cancer or other infectious and inflammatory diseases (Morrison et al., 1984; Boulianne et al, 1984; Carter, 2001). A wide variety of redesigned mAbs of minimal antigen binding fragments provide novel reagents for immunotherapy, medical imaging and immunodiagnostics (Maynard and Georgiou, 2000). Essentially, scFv where VH and VL domains are connected via flexible polypeptide, have been shown to retain the specific monovalent antigen binding affinity of the parent antibody with improved pharmacokinetics for tissue penetration (Bird et al., 1988; Huston et al., 1988; Brinkmann et al., 1995). However, influences of linker size are difficult to predict without the knowledge of the 3-dimensional structure of the recombinant proteins in question.